Method for making an optical fiber chemical sensor comprising a colored indicator, useful in particular for measuring nitric acid

ABSTRACT

The invention concerns the production of a chemical sensor which can be used to measure nitric acidity. 
     This sensor is produced using a sol-gel method for depositing a porous film, containing a colored indicator, on the core of an optic fiber. The pH of the initial sol is adjusted as are other conditions for implementing the sol-gel method to obtain stability of the signal (curves  1  to  4 ) emitted by the sensor in an 8N nitric medium for at least 1000 hours.

TECHNICAL FIELD

The present invention concerns the production of optic fibre chemicalsensors used in particular to measure nitric acidity.

In the area concerning the treatment of spent nuclear fuel, demands interms of quality and process control require very swift knowledge, evenreal-time knowledge, of any variations in physical or chemicalparameters, in nitric acidity in particular.

During the various steps in the treatment of spent fuel, the on-linemeasurement of free acidity provides important data which largelycontributes to control over extraction methods, to a substantialreduction in waste and to a lighter work load for laboratories.

PRIOR ART

Optic fibre chemical sensors suitable for measuring nitric acidity havebeen described by M. H. Noiré et al in the following documents:

-   -   Sensors and Actuators B51, 1998, pages 214-219 [1], and    -   Journal of Sol-Gel Science and Technology 17, 2000, pages        131-136 [2].

These sensors measure the absorbency of a coloured indicator sensitiveto the protons released by the acid. The coloured indicator, ChromoxaneCyanine R for example, is immobilized on a porous film chemicallygrafted onto the core of a silica optic fibre. The optic fibre chemicalsensor is coupled to a spectrophotometric device for remote, in situanalysis of the acidity. The sensor operates by total mitigatedreflection. When this sensor is used, the rays from a light sourcepropagate under multiple reflection in the core of the optic fibre,transiting however along a wave length fraction in the porous filmcontaining the coloured indicator whose colour relates to the acidity ofthe medium with which it is in contact. The transmitted light,representing the acidity of the medium, is measured by visibleUV-spectrophotometry.

The method used for the manufacture of these sensors uses the sol-geltechnique, a soft mode chemical technique for the synthesis of metallicoxides. This technique consists of preparing a sol by acid-catalysedhydrolysis of an alcoxysilane-in-alcohol solution containing thecoloured indicator, leaving the sol to mature to initiate gelling,followed by its depositing on the core of an optic fibre whosemechanical and optic sheaths have been removed over a central part, andthen drying to form a micro-porous film, containing the colouredindicator, grafted onto the core of the fibre.

In this method, the organic precursor, tetraethoxysilane, throughhydrolysis and condensation leads to the formation of an inorganicnetwork of low porosity in which the molecules of the coloured indicatorare trapped.

To conduct acidity measurement, the optic fibre provided with thisporous film is placed in a cell in which the medium to be measuredcirculates, and which is connected via optic fibres to a multi-channelspectrophotometric system fitted with a CCD detector.

The advantage of such device is the possible simultaneous follow-up ofacidity at different points of an installation via several sensors.

The absorbency measured by the detector represents the protonated formof the coloured indicator and is directly linked to the nitric acidconcentration of the medium being analysed.

The sensors manufactured to date using this method show analyticalperforming capacities of interest, but have the disadvantage that theydo not have good reproducibility and especially lifetime characteristicsowing to desorption of the coloured indicator molecules from the porousfilm towards the medium to be analysed.

DESCRIPTION OF THE DISCLOSURE

The subject matter of the present invention is precisely a method formanufacturing an optic fibre chemical sensor using the sol-geltechnique, which has very high stability, that is to say the capacity towithhold practically the entirety of the coloured indicator in theporous film over very long periods, while allowing the protons releasedby the acid to diffuse inside this film.

According to the invention, the method of producing a silica-based opticfibre chemical sensor, which can be used to analyse a chemical speciespresent in a liquid or gas, consists of chemically grafting onto thecore of the optic fibre a porous film containing a coloured indicatorsensitive to the chemical species to be analysed, by conducting thefollowing steps:

-   -   a) Preparing a sol by acid-catalysed hydrolysis of a solution of        an alcoxysilane in an alcohol, containing the coloured        indicator,    -   b) sol maturing    -   c) depositing the sol on the core of the optic fibre, and    -   d) drying    -   and is characterized in that, in step a), the quantity of acid        used is such that the pH of the aqueous phase of the sol is 0.44        to 0.72.

In the sol-gel technique, the choice of parameters used is of greatimportance since these parameters have a direct influence on the finalstructure of the porous film which is to withhold the colouredindicator.

According to the invention, optimum conditions are chosen to obtainporosity providing total withholding of the coloured indicator whileallowing the species to be analysed, for example the protons released bythe acid, to diffuse inside the film.

It was therefore found that the pH of the aqueous phase of the sol is adeterminant parameter for obtaining the desired microporouscharacteristics of the grafted film containing the coloured indicator.

The choice of this parameter has an important influence on the gellingtime of the sol and on the pore sizes of the dried product subsequentlyobtained. Hence, with pH values for the aqueous solution of less than0.72, pores in the microporous region are obtained whereas pH values ofmore than 0.72 lead to the macroporous domain.

Obtaining a macroporous film is not desirable, since it allows thecoloured indicator molecules to diffuse in the solution to be analysedand is detrimental to the reproducibility and lifetime of the sensor.

The pH of the aqueous phase of the sol is generally adjusted to thedesired value through the addition of hydrochloric acid. A value of 0.72relates to a HCl percentage of 2%, 2% meaning 2 moles of HCl per 100moles of alcoxysilane.

Another important parameter when adjusting the porosity of the graftedfilm to the desired values concerns sol maturing step b). Preferably,this maturing is conducted at a temperature of 40 to 70° C., preferablybetween 50 and 60° C., for a period of no more than 3 days.

Indeed, it was verified that maturing time has an influence on the poresize of the dried film. Pore diameter increases with maturing time.Therefore, to limit this diameter, it is appropriate to choose amaturing time which does not exceed 3 days and is preferably between 24and 50 hours.

A further parameter having a considerable influence on the quality ofthe porous film is the quantity of water used for hydrolysis during stepa) for preparation of the sol. Preferably, a water/alcoxysilane molarratio of 4 to 6 is used for hydrolysis. By choosing thiswater/alcoxysilane molar ratio, it is possible to stabilize the densityof the film and its porosity characteristics.

In the method of the invention, to carry out step d), vacuum drying ispreferred for a time of 20 to 30 hours, preferably for approximately 24hours. Drying temperature may be 100° C.

According to the invention, the sensor is preferably used after beingstored for at least 3 weeks away from light and at room temperature.

This additional storage step also has its importance since it allows forstabilisation of the deposited film. Drying at 100° C., even whenconducted on thin films, does not permit total condensation of thealcoxy functions. Therefore the film undergoes change in time throughslow condensation of the remaining alcoxy functions. It is thereforemost important to use the sensor after this condensation process iscompleted so as to avoid encountering reproducibility problems forcharacterization.

Preferably, storage is made for a period ranging from 3 weeks to 2months. It is also possible to accelerate this condensation process bymeans of vacuum drying.

The other parameters of this sol-gel manufacturing method have lesserinfluence on the porosity of the deposited film, and may be chosen fromamong the values chosen for known production methods of chemical sensorsusing the sol-gel technique.

Therefore, the alcohol content of the alcoxysilane solution may be suchthat the alcohol/alcoxysilane molar ratio is approximately 10.

Generally, the alcoxy groups of the alcoxysilane have from 1 to 4 carbonatoms. Preferably tetraethoxysilane is used.

The alcohol used may be an alcohol having from 1 to 4 carbon atoms;preferably ethanol is used which is the most suitable for the synthesisof microporous gels.

In the method of the invention, the coloured indicator is chosen inrelation to the chemical species to be analysed by the chemical sensor.If this sensor is intended to measure nitric acidity over theconcentration range of 1 to 10 mol/L, the coloured indicator may beChromoxane Cyanine R or Chromazurol S. Preferably, Chromoxane Cyanine Ris used.

If the sensor is intended to measure nitric acidities over a lowerconcentration range, for example from 0.1 to 2 mol/L, the colouredindicator may be chosen from among Thymol Blue, Phenol Red andPyrocatechol Violet.

The concentrations of the coloured indicator are chosen such that asufficient quantity of indicator is obtained in the film. They may besuch that the coloured indicator/alcoxysilane molar ratio ranges from{fraction (1/300)} to {fraction (1/700)}. It is preferably 1:335. Athigher values, dimers and/or aggregates may occur.

A further subject of the invention is a fibre optic chemical sensor formeasuring nitric acidity, obtained using the above method, whose aciditymeasurement signal is stable for at least 1000 hours in 8 N acidcirculated around the porous film.

Other characteristics and advantages of the invention will become betterapparent on reading the following description of examples of embodimentgiven by way of illustration and evidently non-restrictive, withreference to the appended drawings.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the sol-gel method used for theinvention.

FIG. 2 is a diagram illustrating a measurement installation comprising achemical sensor conforming to the invention.

FIG. 3 shows the absorption spectra of a sensor conforming to theinvention, in a pure nitric medium, for acidity values ranging from 2 to10 N relative to a 1 N reference.

FIG. 4 illustrates changes in the signal transmitted by differentsensors in relation to time in hours, curves 1 to 4 relate to sensorsconforming to the invention, whereas curves 5 to 8 are given by way ofcomparison and represent sensors which do not conform to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a sol-gel method for producing a silica-based porous filmcontaining a coloured indicator formed of Chromoxane Cyanine R (CCR).

As illustrated in this figure, the starting alcoxysilane istetraethoxysilane Si (OEt)₄ in solution in ethanol EtOH, to which isadded water H₂O and an acid catalyst, hydrochloric acid HCl, and acoloured indicator CCR.

Hydrolysis leads to Si(OH)₄ which, by condensation, gives a sol in whichthe CCR molecules are trapped. By sol maturing, a gel is obtained asshown in this figure. Generally, the sol-gel matrix is prepared at roomtemperature in a clean environment protected from draughts and, ifpossible, under controlled temperature and hygrometry.

To implement the method of the invention, an optic fibre such as asilica fibre may be used comprising an optic sheath in hard polymer andan outer sheath in Tefzel, having a total length of 256 mm. The centralpart of this fibre, or active part, is uncovered, over 100 mm forexample, to expose the core of the fibre. It is possible to conduct afirst mechanical removing operation to remove the outer Tefzel sheath,and a second removal operation under heat to remove the optic sheath ofhard polymer. Sol-gel depositing is then carried out on the active partof this fibre previously cleaned with ethanol for example.

This depositing may be made by placing the fibre vertically in a tubecontaining the sol, then by withdrawing it vertically at a slow,constant rate, for example at 1 mm/s. The ends of the mechanical sheathimmersed in the sol-gel solution are then cleaned with alcohol. Afterdepositing, the coated fibre is dried, for example at a temperature of100° C., so that the film adheres to the fibre and porosity is reduced.

This depositing step is conducted away from any draughts of air toobtain a uniform deposit thickness when the solvent is evaporated It isalso possible to conduct several successive deposits by immersing thefibre in the sol to obtain the desired thickness.

The following examples illustrate the preparation of the sensors usingthe method of the invention.

EXAMPLE 1

Sensors 1 and 2 are prepared from two identical sols, obtained bysuccessively adding to a sealed flask in opaque glass: absolute ethanol,99% pure tetraethoxysilane (TEOS), dilute hydrochloric acid and thecoloured indicator CCR having a molecular weight M of 536.4 and 40%purity.

For this preparation, the quantity of hydrochloric acid used is suchthat the pH of the aqueous phase of the sol is 0.72, the water/TEOSmolar ratio is 6, the ethanol/TEOS molar ratio is 10 and CCRconcentration represents 1 mole CCR per 335 moles TEOS.

The mixture is homogenized for 1 hour at room temperature, and it isthen placed in sealed storage in an oven at 55° C. for a maturing timeof 50 hours, before the sol is deposited on the fibre using theabove-described method.

After depositing the film, vacuum drying is conducted at 100° C. for 24hours and the sensor is then stored for 3 weeks in ambient atmosphere.

EXAMPLE 2

The same operating mode is followed as in example 1 to prepare sensors 3to 8, using the same parameters for the method except those concerningthe pH of the aqueous phase of the sol and temperature.

Table 1 below illustrates the values chosen for the pH of the aqueousphase and maturing temperature to prepare sensors 1 to 8.

TABLE 1 Sensor pH Temperature (° C.) 1 0.72 55 2 0.72 55 3 0.44 42 40.44 68 5 0.99 42 6 1.27 55 7 0.17 55 8 0.99 68

Sensors 1 to 4 prepared as described above, have the followingcharacteristics:

-   -   The thickness of the film is in the region of 100 nm for a sol        layer deposit. This thickness is measured on optic fibre by        scanning electronic microscopy (SEM) and on silicon plate by X        reflectometry and ellipsometry.    -   The density of the film is 1.85 g.cm⁻³ measured by X        reflectometry. The deduced porous volume is 16%.    -   The refractive index of the film is 1.44 compared with 1.46 for        the index of the optic fibre core in melted silica. This value        was obtained by ellipsometry on silicon plate.

The chemical sensors thus obtained were tested in 8 N nitric medium.

For this purpose, the device shown in FIG. 2 is used.

This device comprises an xenon lamp 1 which sends a light beam ontosensor 3 in contact with the medium to be measured and onto a referenceline 5 measuring possible fluctuations in the lamp signal. The lightbeams are then directed by optic fibres 7 into a plane fieldspectrophotometer 9 fitted with mirrors and a Charge Coupled Devicedetector (CCD) which is a two-dimension matrix detection system, thecolumns representing wavelengths and the lines representing the positionof the 10 fibres (or 10 measurement channels) as seen by the detector.

The absorption spectra of the sensor in 1N HNO₃ medium are obtainedwhich is the reference, and the spectra corresponding to the sensors in8 N HNO₃ medium, i.e. the measurement. The optic density is determinedon these absorption spectra at the wavelength of maximum absorptionlocated at 545 nanometres, compared with the reference which is 1 Nnitric acid.

FIG. 3 shows the absorption spectra obtained for nitric acidconcentrations of 2, 5, 8, 10 and 12 N.

The stability of the signal emitted by each of sensors 1 to 8 in anitric medium is verified by measuring the optic density at time t_(o),which is 0.14 for 8 N HNO₃ and which corresponds to 100% of the signal;then the optic signal is determined in relation to time by itsexpression as a percentage of initial optic density, measured for theconcentration HNO₃=8 N.

The results obtained are shown in FIG. 4 which illustrates the changesin the signals emitted by sensors 1 to 8 in relation to time (in hours).

In this figure, it can be seen that the best results are obtained withsensors 1 to 4 produced with a pH of 0.72 or less, and that sensors 5and 7 also correspond to average stability.

On the other hand, sensors 6 and 8 show no signal stability.

Therefore, it is verified that the choice of parameters such as pH,temperature and maturing time, according to the invention, play a veryimportant role in results, in particular in respect of sensor stability.

The response of the sensors of the invention was also measured in thepresence of metallic cations such as FE³⁺, Ce³⁺, UO₂ ²⁺, Pu (IV), U (IV)and it was verified that for contents of these elements lower than 10g.L⁻¹, comparable results were obtained.

Cited References

[1] M. H. Noiré et al, Sensors and Actuators B51, 1998, pages 214-219.

[2] M. H. Noiré et al, Journal of Sol-Gel Sciences and Technology 17,2000 pages 131-136.

1. Method for producing a silica-based optic fibre chemical sensor whichcan be used to analyse a chemical species present in a liquid or gas,consisting of chemically grafting onto the core of the optic fibre aporous film containing a coloured indicator sensitive to the chemicalspedies to be analysed, by conducting the following steps: a) preparinga sol by acid-catalysed hydrolysis of a solution of an alcoxysilane inan alcohol containing the coloured indicator, b) maturing to sol, c)depositing the sol on the core of the optic fibre, and d) dryingcharacterized in that in step a), the quantity of acid used is such thatthe pH of the aqueous phase of the sol lies between 0.44 and 0.72. 2.Method according to claim 1, in which for step b) the sol is matured ata temperature of 40 to 70° C. for a time of no more than three days. 3.Method according to claim 2, in which the maturing temperature is 50 to60° C.
 4. Method according to claim 2, in which the sol maturing time isfrom 24 to 50 hours.
 5. Method according to claim 1, in which thehydrolysis in step a) is conducted using a water:alcoxysilane molarratio of 4 to
 6. 6. Method according to claim 1, in which for step d)drying is conducted in a vacuum for 20 to 30 hours.
 7. Method accordingto claim 6, in which the drying time is approximately 24 hours. 8.Method according to claim 1, in which the sensor is also stored for atleast three weeks before use.
 9. Method according to claim 1, in whichfor step a) a solution of tetraethoxysilane in ethanol is used having anethanol:tetraethoxysilane molar ratio of approximately
 10. 10. Methodaccording to claim 5, in which for step a) a solution oftetraethoxysilane in ethanol is used having an ethanol:tetraethoxysilanemolar ratio of approximately
 10. 11. Method according to claim 1, inwhich the coloured indicator/alcoxysilane molar ratio is 1:335. 12.Method according to claim 1, in which the species to be analysed isnitric acid in the concentration range of 1 to 10 mol.L⁻¹.
 13. Methodaccording to claim 12, in which the coloured indicator is ChromoxaneCyanine R.
 14. Method according to claim 12, in which the colouredindicator is Chromazurol S.
 15. Method according to claim 1, in whichthe species to be analysed is nitric acid in the concentration range of0.1 to 2 mol.L⁻¹.
 16. Method according to claim 15, in which thecoloured indicator is chosen from among Thymol Blue, Phenol Red andPyrocatechol Violet.